Register fd to topic when IncreaseTopicCount

Insert fd to topic's fd set for the monitor of clients
Add command clientsof in Redis procotol
Modify variable name to satisfy google-cpp-style.

topic_lock -> topic_lock_
host_lock -> host_lock_
clients -> clients_
Rearrange code in UpdateTopicTable.

Use the topic from topic_table_ in monitor thread instead of worker thread
Merge active_topic fds_by_topic topic_count into topic_stats
Merge pull request #24 from Shanicky/monitor

Monitor the clients of each topic.